A common problem on electrical power distribution systems supplying power to inductive loads is the need to provide reactive power compensation. Large motors and other types of inductive loads used, for example, in lumber mills, rock crushing plants, steel mills, and to drive elevators and pumps, shift the power factor of the system away from the desired unity level, thereby decreasing the efficiency of the power system. Compensation for the effects of inductive loads can be provided to control line voltage, power factor, or volt-ampere-reactive (VAR) power. Such compensation generally takes the form of capacitor banks that are connected to transmission and distribution lines. While an appropriate capacitive compensation can be determined and left on-line to compensate continuously running inductive loads, most inductive loads operate intermittently and cyclically, requiring that the correct compensation be selectively applied in response to a varying reactive load on the system. Mechanical contactors are typically employed to connect and switch the capacitor banks to compensate changing inductive loads. However, mechanical contactors are known to introduce undesirable transients each time that they operate to change the reactive compensation. Furthermore, being mechanical devices, mechanical contactors must be maintained, rebuilt, or even replaced after a limited number of operating cycles.
Other devices have been developed for controlling reactive power at the point of use, such as a power factor control system for induction motors developed by Frank J. Nola, which is described in U.S. Pat. No. 4,266,177. Unfortunately, there are several problems with the Nola control system that have prevented it from being widely used. For example, operational parameters of this type of device must be tailored for use with a specific inductive load. In addition, the Nola device can control power factor over only a limited range restricted to light loads, since the device will not work at full loads. Three Nola devices are required to control the power factor of a three-phase inductive load.
A reportedly transient-free, solid state automatic power factor correction apparatus is disclosed in U.S. Pat. No. 4,645,997. This apparatus is designed to automatically correct power factor in a multi-phase system, on the load side of a distribution transformer, e.g., to correct the power factor of an inductive load within a plant. It generates signals indicative of the voltage and current associated with each phase supplying power to the load. The current and voltage signals for each phase are compared to each other to determine the extent of current lag, and a signal indicative of current lag is generated for each line. A microprocessor-controlled circuit converts these signals into a lagging phase angle in degrees and determines the cosine of the angle and thus, the power factor of the line. The microprocessor also controls a switching network that is capable of selectively adding or removing banks of delta-connected capacitors to or from the power lines to control power factor. Only two silicon-controlled rectifiers (SCRs) comprise the switching network for each bank of capacitors. According to this patent, the SCRs can connect the capacitor banks to the lines at any time, regardless of the voltage on the capacitors, without creating current surges or electrical transients. However, this device is intended to operate at relatively low distribution transformer secondary voltage levels typically used in a plant i.e., 480 volts or less, and it cannot accurately determine the required reactive power compensation to control power factor if there are significant harmonics of the fundamental 60 Hz line frequency present in the current or voltage, since such harmonic distortion interferes with the measurement of the phase angle or lag time between voltage and current. In addition, the system disclosed in U.S. Pat. No. 4,645,997 lacks other features required for fully automated, unattended operation, such as the capability for remote control of the switching network and the ability to detect and compensate for malfunctions in the device. In any case, it is generally more effective for an electric utility to provide reactive power compensation on the lines of an electric power distribution system rather than depending on the customer to correct each load. The device disclosed in this patent cannot be used on distribution lines, because it cannot be controlled remotely, cannot operate unattended, and cannot operate at the higher voltages typically used on distribution lines (up to 35 KV).
In U.S. Pat. No. 4,645,364, which is issued to Williams and two of the inventors of the present invention, a reactive power compensating system is disclosed that is designed to directly compensate an inductive load on multi-phase lines of a distribution system. The apparatus includes fixed capacitors that are always connected to each phase of the system to provide a minimum reactive power compensation, and selectively switched capacitors that are connected to each phase by a solid state switching network of SCR and diode pairs to provide any additional compensation required. The required reactive power compensation for all phases is determined in the device by sensing the current on only one phase at the time its voltage crosses zero.
The reactive power compensation system described in the Williams et al. patent is deficient in several important respects. Since only one phase of a multi-phase distribution system is used to determine the required reactive power compensation for all of the phases, the system cannot properly compensate for different inductive demands on each of the phases resulting, for example, from various single phase and multi-phase inductive motors. Also, the determination of the required reactive power compensation is susceptible to errors caused by harmonic distortion in the line current and voltage--a problem that is specifically admitted in the patent. In some situations, use of a fixed capacitance to provide a minimum required compensation may be inappropriate, since, if all of the significant inductive loads connected to the system are at times de-energized, the correct compensation may be much less than that provided by the fixed capacitance. Furthermore, fault tolerance and operation of the switching network in the presence of voltage transients and harmonics are not addressed in this reference.
In consideration of the above-noted problems that exist with the prior art systems used to compensate for inductive loads, a reactive power compensation system is herein described that compensates for different inductive loads on each phase. It is an object of the present invention to provide a switch and a method for selectively connecting an appropriate capacitance to provide the compensation required for the inductive load on each phase of an electrical power distribution system. More generally, it is an object of the present invention to provide an optically triggered solid state switch and a method for selectively controlling the flow of an electrical current at a high voltage without introducing transients or harmonic distortion in that electrical current. These and other objects and advantages of the present invention will be apparent from the attached drawings and from the Description of the Preferred Embodiments that follows.